DVT occurs when a clot forms in leg
veins during periods of relative
immobility, such as long-haul travel.
The clot can then migrate to the lungs
in what is called a pulmonary embolism
(PE), or the heart or brain, causing a
heart attack or stroke.

The overall phenomenon is known as
venous thromboembolism (VTE), the
World Health Organization said in its
project titled Research into Global
Hazards of Travel.

Following five separate research
studies as part of first phase of the
project, the WHO said that “the risk
of developing venous thromboembolism
approximately doubles after travel
lasting four hours or more.”

The absolute risk of developing VTE if
seated and immobile for more than four
hours remains “relatively low” at
about one in 6,000, according to the
WHO.

However, the chances are greater for
people deemed at risk, for example,
those who regularly take long-haul
flights, those who are obese, either
very tall or very short (above 1.9
meters and below 1.6 meters, or above
6 feet 3 inches and below 5 feet 3
inches), women who take oral
contraception, or people with
inherited blood disorders.

“It is a public health issue,” said
Shanthi Mendis, WHO’s senior adviser
on cardiovascular diseases, noting
that about 2 billion people are
estimated to fly every year.

The WHO stressed that the risks posed
by immobility do not just apply to
airline passengers but also those who
undertake long journeys by car, bus or
train.

According to the World Health
Organization, travelers can take
“commonsense” measures to reduce the
risks of contracting VTE by regularly
exercising their calf and ankle
muscles during a journey, or simply
getting up and moving around a little.

Passengers should also avoid taking
sedatives or too much alcohol ahead of
a voyage.

However, the WHO did not make any
specific recommendations on prevention
such as blood-thinning medications and
elastic stockings which seek to
prevent clotting, saying more research
was needed into possible side-effects.

According to the WHO, there is a clear
need for travelers to be given
appropriate information regarding the
risks. Further studies to identify
preventive measures will be done under
the second phase of the research
project.